1. Field of the Invention
The present invention generally relates to photovoltaic cells and, more particularly, to an improved photovoltaic cell having a passivation structure that results in improved performance and efficiency.
2. Description of Related Art
The interest in photovoltaic (PV) cells continues as concerns over pollution and limited resources continue. The continued interest has been in both terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, the concern over limited resources of any type is a major one. This is because the need to increase the amount of a resource increases the payload. And an increased payload can increase the cost of a launch more than linearly. But with the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy is an obvious alternative to increased payload. Irrespective of the application, and as with any energy generation system, efforts have been ongoing into increasing the output and/or efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight. The stacked arrangement has been provided in a monolithic structure on a single substrate or on multiple substrates. Examples of multi-cell devices are shown in U.S. Pat. Nos. 5,800,630; 5,407,491; 5,100,478; 4,332,974; 4,255,211; and 4,017,332.
In the multiple cell device, semiconductor materials are typically lattice matched to form multiple p-n (or n-p) junctions. The p-n (or n-p) junctions can be of the homojunction or heterojunction type. When solar electromagnetic energy is received at a junction, excess charge carriers (i.e., electrons and holes) are generated in the conduction and valence bands in the semiconductor materials adjacent the junction. A voltage is thereby created across the junction and a current can be utilized therefrom. As the solar electromagnetic energy passes to the next junction which has been optimized to a lower energy range, additional solar energy but at this lower energy range can be converted into a useful current. With a greater number of junctions, there can be greater conversion efficiency and increased output voltage.
But for the multiple cell PV device, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions and other conductive interfacing layers have been used to minimize the blockage of current flow. In a multiple wafer structure, metal grids or transparent conductive layers have been used for low resistance connectivity.
Another limitation to the multiple cell PV device is that current output at each junction must be the same for optimum efficiency in the series connection. Also, there is a practical limit on the number of junctions, since each successive junction incurs losses compared to a theoretical maximum.
The concern over efficiency in PV cells has created more interest in the use of germanium, gallium arsenide, indium phosphide, and gallium indium phosphide, which tend to be more efficient than their silicon predecessor. Indium phosphide, and phosphide semiconductors in general, have another advantage of being radiation resistant, which is of particular benefit in space applications.
Whether in the multiple junction or single junction PV device, a conventional characteristic of PV cells has been the use of a single window layer on an emitter layer disposed on a base/substrate, which is shown for example in U.S. Pat. No. 5,322,573. Alternatively, the single window layer is directly disposed on the base/substrate. In either instance, the single window layer serves as a passivation layer whereby minority carrier recombination is sought to be reduced at the front surface of the emitter layer (or base where there is no emitter layer). The reduction in surface recombination tends to increase cell efficiency.
Additionally, since the single window layer needs to be light transmissive, the single window layer has typically been relatively thin, i.e., less than about 100 nm thick and, for example, in the range of about 10 to 100 nm thick, as shown in U.S. Pat. No. 5,322,573. In fact, the past art has taught that the window layer should be as thin as possible, such as in U.S. Pat. No. 5,322,573. However, some of the disadvantages of the single window layer used in the past include the fact that minority carrier surface recombination can still occur to an extent that negatively impacts performance. Further, a relatively thin window layer may not present a satisfactory barrier to unwanted diffusion of impurities, which can produce shunts or crystalline defects. A thin window also provides less mechanical strength, smaller sheet conductivity to supplement the emitter in reducing lateral conductivity losses, and may not allow sufficient freedom to minimize reflectance losses.
Similar to the conventional use of single layer window has been the use of a single layer back-surface field structure below the base/substrate, as shown in U.S. Pat. No. 5,800,630. The typical purpose of the back-surface field structure has been to serve as a passivation layer like the single window layer described above. However, the disadvantages of the single layer back-surface field structure include those mentioned above with respect to a single window layer.
As can be seen, there is a need for an improved photovolaic cell, including one that can be incorporated into a multi-junction or single junction device. Also needed is an improved photovoltaic cell that is of a heterojunction type and can include either a p-n or n-p junction. Another need is for a photovoltaic cell that has increased efficiency and greater output voltage. A further need is for an improved passivation structure that can be used on a photovoltaic cell and, more generally, on a minority-carrier semiconductor device. Also needed is an improved passivation structure that can be used on a photovoltaic device that either contains or does not contain an emitter layer and/or back-surface field structure. A photovoltaic cell with a base made from Ge, GaAs, or GaInP is also needed with better efficiency.